As a former teacher, I 100% agree with you. Whenever I was explaining the same stuff 4 times in a row in 4 different classes, I thought "this would be so much more efficient with video". With the advantage that sick kids that stayed at home could watch the video, too. Or anyone who didn't pay attention could watch the video again before exams. And their parents could watch the video too, if they are curious.
And if more teachers would do it like this, and they would put their videos online, it means that if your teacher's explanation didn't work for you, for whatever reason, you can try another one. Having the same thing explained from 2 or 3 different angles would probably help.
The usual response is: "Teacher's job isn't just explaining; it's also making sure the kids understood properly, explaining questions, giving problems, checking the answers to the problems, etc." Yeah, so we could have 1/2 of the lesson on video, and another 1/2 solving problems and answering questions. Still an improvement over how things are done now. And a part of "giving problems, checking solution" could be done by a computer, by the way.
The part of the response I agree with, is that you cannot eliminate the helpers completely. No matter how well the videos would explain it, some students would misunderstand it creatively, and if there is no one to notice and fix the problem, the outcomes will be bad for a fraction of students. Also, for kids, someone needs to be there to make sure they actually watch the videos, and don't play with their phones during the lesson instead.
Khan Academy is fantastic! My daughter loves it, but she also sometimes gets stuck on a lesson, and needs to ask me. In principle, this could be solved by collecting feedback, and creating optional extra slow explanations for people who got stuck at some point. Like: if you didn't understand the short video, watch this long one.
The way we do teaching now is just... dunno, pretending that we live in 19th century and can't do better. It still works, but has a lot of problems. For example, if your teacher sucks, and you can't change schools easily, you are screwed; and I have heard stories about incompetent teachers.
(Also, all textbooks for elementary and high school should be open-source, and freely downloadable from one web page. With the option for volunteers to fix mistakes, add more explanations, make translations, etc.)